BibTeX:
@incollection{Various,
  author = {Various},
  title = {The Comforter of the Faithful: Bahíyyih Khánum},
  booktitle = {Bahíyyih Khánum: The Greatest Holy Leaf},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/ebooks/19242},
}

APA:
Various. The Comforter of the Faithful: Bahíyyih Khánum. In *Bahíyyih Khánum: The Greatest Holy Leaf*. Bahá'í World Centre.

MLA:
Various. "The Comforter of the Faithful: Bahíyyih Khánum." *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, n.d..

Chicago:
Various. "The Comforter of the Faithful: Bahíyyih Khánum." In *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, n.d..